Decadal Variation of the Northern Hemisphere Annular Mode and Its Influence on the East Asian Trough

584 JOURNAL OF METEOROLOGICAL RESEARCH VOL.30 Decadal Variation of the Northern Hemisphere Annular Mode and Its Influence on the East Asian Trough LU Chunhui 1 ( ), ZHOU Botao 1,2 ( ), and DING Yihui 1 ( ) 1 Laboratory for Climate Studies, National Climate Center, China Meteorological Administration, Beijing 100081 2 Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters, Nanjing University of Information Science & Technology, Nanjing 210044 (Received December 3, 2015; in final form April 19, 2016) ABSTRACT We analyze the decadal variation of the stratosphere troposphere coupled system around the year 2000 by using the NCEP reanalysis-2 data. Specifically, the relationship between the Northern Hemisphere Annular Mode (NAM) and the tropospheric East Asian trough is investigated in order to find the effective stratospheric signals during cold air outbreaks in China. Statistical analyses and dynamic diagnoses both indicate that after 2000, increased stratospheric polar vortex disturbances occur and the NAM is mainly in negative phase. The tropospheric polar areas are directly affected by the polar vortex, and in the midlatitudes, the Ural blocking high and East Asian trough are more active, which lead to enhanced cold air activities in eastern and northern China. Further investigation reveals that under this circulation pattern, downward propagations of negative NAM index are closely related to the intensity variation of the East Asian trough. When negative NAM anomalies propagate down to the upper troposphere and reach a certain intensity (standardized NAM index less than 1), they result in apparent reinforcement of the East Asian trough, which reaches its maximum intensity about one week later. The northerly wind behind the trough transports cold air southward and eastward, and the range of influence and the intensity are closely associated with the trough location. Therefore, the NAM index can be used as a measure of the signals from the disturbed stratosphere to give some indication of cold air activities in China. Key words: decadal variation, Northern Hemisphere Annular Mode, East Asian trough, cold air activities Citation: Lu Chunhui, Zhou Botao, and Ding Yihui, 2016: Decadal variation of the Northern Hemisphere Annular Mode and its influence on the East Asian trough. J. Meteor. Res., 30(4), 584 597, doi: 10.1007/s13351-016-5105-3. 1. Introduction The stratosphere is a neutral atmospheric layer located between the troposphere and mesosphere, from 10 to 55 km above sea level. Although the stratosphere only accounts for 15% of the total mass of earth s atmosphere, an increasing amount of research has shown that stratospheric processes and their interactions with the troposphere play an important role in the whole climate system (Thompson and Wallace, 2001; Thompson et al., 2002; Baldwin et al., 2003; Cai and Ren, 2007). The study of the influences of stratospheric anomalies on the troposphere can be traced back to 1977 (Quiroz, 1977), while systematic investigations and analyses have appeared in recent years and concentrated on the Arctic Oscillation (AO), which refers to the first Empirical Orthogonal Function (EOF) mode of sea level pressure (Thompson and Wallace, 1998). Baldwin and Dunkerton (1999) found that spatial structures similar to the AO not only exist within the troposphere, but also extend upward into the stratosphere. Furthermore, with increasing height, the distribution of the AO in the midlatitudes develops gradually into a spatial distribution of the annular mode, and thus they proposed the concept of the Northern Hemisphere Annular Mode (NAM) (Wallace, Supported by the National Natural Science Foundation of China (41275078 and 41205041), National Key Research and Development Program of China (2016YFA0600701), and China Meteorological Administration Special Public Welfare Research Fund (GYHY201306026). Corresponding author: zhoubt@cma.gov.cn. The Chinese Meteorological Society and Springer-Verlag Berlin Heidelberg 2016

NO.4 LU Chunhui, ZHOU Botao and DING Yihui 585 2000). Later, Baldwin and Dunkerton (2001) calculated an NAM index and used it to composite polar vortex events into 18 weak and 30 strong cases. These composite results clearly show the propagation of NAM signals from the upper stratosphere down to the troposphere. Therefore, the NAM index can be used as an important indicator to reflect variation processes of the stratospheric circulation and interactions between the stratosphere and troposphere. Decadal variation is one of the most important characteristics of atmospheric circulation, and considerable research has been conducted on decadal variation in the troposphere (Sun and Wang, 2006; Meehl et al., 2013; Wu and Zhang, 2015). From climate change studies in the 20th century, it has been uncovered that there was a decadal disturbance in the atmospheric teleconnection patterns in the midlatitudes of the Northern Hemisphere around the 1970s (Wang, 2001; Gong and Ho, 2002), and Graham et al. (1994) considered that this was related to El Niño Southern Oscillation activity. Later, Si et al. (2009) and Si and Ding (2012) found that the Meiyu rainfall in China exhibited a decadal oscillation in the late 1990s as well, and after 1999 the rainbelt moved northward to the Yellow River valley. However, because of the lack of reliable and long-term stratospheric data, the study of decadal variation characteristics of the stratosphere remains limited. In analyses of stratospheric sudden warming (SSW) (Lu and Ding, 2013), it was found that the stratospheric polar vortex had more disturbances in boreal winter after the turn of the 21st century. During the 12 winters after the year 2000, there were 10 SSW events, and strong circulation anomalies with split polar vortex features appeared 6 times. In contrast, during the 21 winters from 1979 to 2000, only 11 SSW events were observed and most of these were weak warming events that displayed the pattern of vortex displacement. Diagnoses of the isentropic potential vorticity (Lu and Ding, 2015) further proved the above results. Seven typical weak polar vortex events were observed after 2000, and among these the SSW process that occurred in the 2009/10 winter had important impacts on the activity of cold air in eastern China. It is as yet uncertain whether this may indicate decadal variation in the stratospheric circulations around 2000, and whether this stratospheric change is able to influence the stratosphere troposphere coupled system. In this study, we first investigate the decadal variation characteristics of the stratosphere troposphere system around 2000 by using statistical analyses. We then diagnose the relationship between the NAM index and the East Asian trough (the most important synoptic system in East Asia in boreal winter), to further understand the influences of stratospheric anomalies on the lower atmosphere and explore the effective stratospheric signals during cold air outbreaks in China. 2. Data and analysis Data from the NCEP reanalysis-2 (NCEP2) are used to investigate the decadal variation of the stratospheric NAM and its influence on the East Asian trough. NCEP2 has 17 levels vertically, from 1000 hpa to the upper stratosphere (10 hpa), and a T159 spectral resolution (Kanamitsu et al., 2002). In this study, we apply a 144 73 (longitude latitude) grid of daily and monthly mean data from 1 January 1979 to 31 December 2012. We define the differences between the original data and their respective climate means as anomalies to analyze the annual and decadal variation characteristics of the stratosphere troposphere coupled system. The multi-year mean of monthly data from 1990 to 2000 represents the decadal mean of the 1990s; and similarly, the multi-year average from 2001 to 2011 is used to denote the decadal mean of the 2000s. The NAM index is one of the most important indicators in describing the variation characteristics of the polar vortex in both the stratosphere and troposphere. The data used here are derived from the National Climate Center (NCC) (http://cmdp.ncc.cma. gov.cn/cn/monitoring.htm) of China. The data are updated in real time. The calculation method used by the NCC is consistent with that of Thompson and Wallace (2001). EOF analyses are conducted on the monthly geopotential height (GH) of the Northern Hemisphere for all extended winters (from September

586 JOURNAL OF METEOROLOGICAL RESEARCH VOL.30 to the following April) from 1958 to 2010, and the spatial distribution of the first eigenvector or the first EOF mode, is obtained. Then, we project the daily GH onto the first EOF mode, level by level, to obtain the time coefficient; after standardization, we obtain the NAM indices of each day at each level. This index starts from January 1981, and has already been applied in some previous investigations (Wan et al., 2013). The East Asian trough is an important synoptic system in the troposphere, and has been defined from many perspectives, such as the regional average from the static point of view (Mu and Li, 2000) and the gradient of meridional wind from the dynamical point of view (Huang et al., 2013). In the current study, the definition of the intensity and position of the East Asian trough given by the NCC is used to carry out dynamic diagnoses and statistical analyses. First, we identify a shape similar to the right-facing arc as the trough line over 35 55 N, 110 170 E at 500 hpa. This line must be smooth and with maximum curvature, and the GH of the line must be the smallest in the corresponding grid box. The mean value of each point s longitude on the trough line is defined as the trough position. The sum of each point s GH on the trough line, subtracted by the maximum value and then with the minimum value added, is defined as the trough intensity. A smaller value indicates a stronger trough, and vice versa. 3. Decadal variation of the stratosphere troposphere system We first carry out an EOF analysis of continuous monthly GH anomalies during 1979 2011. Figure 1 shows distributions of the first EOF mode on different levels. This mode accounts for 31.3% of the total variance. In Fig. 1, the first EOF mode mainly exhibits the distribution characteristics of the NAM. In polar areas, positive GH anomalies are observed in the whole atmosphere from 1000 to 20 hpa, corresponding to a weak polar vortex and the negative phase of the NAM. In the midlatitudes, negative GH anomalies are located around the polar vortex from the middle stratosphere (100 hpa) down to the 1000-hPa level. At the 500- and 1000-hPa levels, large-value centers of negative anomalies are located in the northeast of the Eurasian continent and eastern part of North Atlantic, respectively, while in the Ural Mountains region, positive GH anomalies are observed. These distribution characteristics in boreal winter indicate reinforcement of the Ural blocking high and East Asian trough, which is beneficial for the accumulation and outbreak of cold air. According to the time series of the first EOF mode of GH anomalies (Fig. 2a), most of the major increases or decreases of the time series occur in the period from November to next March, while from April to October the values are quite small, oscillating around zero. This indicates that the first EOF mode mainly reflects the distribution and variation features of the polar vortex in boreal winter. When the time series shows a clear enhancement, positive GH anomalies dominate the polar areas in both the stratosphere and troposphere, corresponding to a weakened polar vortex and increased disturbances in the region. In the 2000s, the time series is mostly positive, and according to Lu and Ding (2013), the stratospheric polar vortex does have more disturbances in boreal winter. These polar anomalies are vertically continuous in the whole atmosphere and can thus directly affect the lower atmospheric circulations. In the tropospheric midlatitudes, GH increases in the Ural Mountains area and decreases in East Asia, which is beneficial to the development of a blocking high and the East Asian trough, leading to enhanced cold air activities in eastern and northern China. In contrast, when the time series is negative, the polar vortex becomes stronger with reduced disturbances, and the intensities of the blocking high and trough noticeably weaken, corresponding to reduced cold air activities in China in boreal winter. Another notable feature in Fig. 2a is that the time series exhibits an apparent increasing trend from January 1989 to December 2011. The red curve shows the distribution of winter mean (November to the next March) time series in these 23 winters. By placing June 2000 as a threshold, it is clear that in the 12 winters from January 1989 to March 2000, the time

NO.4 LU Chunhui, ZHOU Botao and DING Yihui 587 Fig. 1. Distributions of the first EOF mode of the GH anomalies (gpm) at (a) 20, (b) 100, (c) 300, (d) 500, (e) 700, and (f) 1000 hpa in the Northern Hemisphere (20 90 N). series values are mainly negative, and in 6 of the winters during this period, the time series maintains large negative values for a long time. However, in the 11 winters between November 2000 and December 2011, the time series values are mostly positive, and a large variation amplitude is apparent for 5 of the winters. From the evolution of the linear trend curve (black dotted line), a rising trend of the time series in the recent 23 winters can also be clearly observed. These variation features indicate a more stable polar vortex and that the NAM is in positive phases in boreal winter during the 1990s. The intensities of the Ural blocking high and East Asian trough, which have major influences on the cold air activities in China, are also weak. In fact, since 2000, these conditions are toward the opposite. If checking the winter months or early/late winter, the influences on the trend are small and the rising trend of the time series is still obvious. There will be some impacts on the variation amplitude of the winter mean time series, but the main conclusions are the same as above. Furthermore, the temporal evolution of the regional mean GH anomalies in polar areas (60 90 N) at 100 hpa (Fig. 2b) can also prove the characteristics mentioned above. This time series, which reflects variation of the intensity of the stratospheric polar vortex, exhibits a similar distribution to the time series of the first EOF mode (Fig. 2a), with a correlation coefficient of 0.9. The results of the winter running mean and linear trend are also consistent, indicating that there is indeed a decadal variation in the development of the polar vortex from the 1990s to 2000s and its intensity changes from strong to weak. A 20-yr quasi-periodic fluctuation feature is investigated by using bandpass filter analysis (blue dotted line in Fig. 2). Due to the short time span of the reanalysis data, it fails the statistical test and is not statistically significant. To further illustrate the decadal variation of the

588 JOURNAL OF METEOROLOGICAL RESEARCH VOL.30 Fig. 2. (a) Time series of the first EOF mode of the GH anomalies from 1979 to 2011 (grey dashed line) and (b) temporal evolution of the regional mean GH anomalies in polar areas (60 90 N) at 100 hpa (grey dashed line; gpm). The red solid lines indicate results of the winter running mean; the black dotted lines indicate the linear trend from 1989 to 2011; and the blue dashed lines indicate bandpass filtered results. stratosphere troposphere coupled system, Fig. 3 shows the GH differences between the 2000s and 1990s at different pressure levels in the Northern Hemispheric winter. Figure 3 shows that the decadal difference in the polar vortex is continuous in the vertical direction. In the lower stratosphere (100 hpa), positive values of GH differences dominate the polar areas throughout winter, demonstrating that the decadal mean intensity of the polar vortex is weaker in the 2000s than in the 1990s. In early winter, this positive center is still weak and the coverage is small. Over time, this center gradually strengthens and expands to lower latitude regions. It reaches its maximum strength in February and the periphery of the positive center also extends southward to the regions around 60 N. In the midlatitudes, outside the polar vortex, there are two negative centers of GH differences located in the Atlantic areas and the middle and eastern parts of the Eurasian continent, and the intensity of the negative center in eastern Eurasia is stronger. According to Lu and Ding (2013), these GH negative anomalies are closely related to the cold air activity in eastern and northern China. At 300 hpa (the mid and upper troposphere), the distributions of the GH difference between the 2000s and 1990s are similar to those at 100 hpa, and a major exception is

NO.4 LU Chunhui, ZHOU Botao and DING Yihui 589 Fig. 3. Differences of GH (gpm) between the 2000s and 1990s in boreal winter at (a, b, c) 100, (d, e, f) 300, and (g, h, i) 500 hpa, in (a, d, g) December, (b, e, h) January, and (c, f, i) February. Areas within white contour lines contain statistically significant anomalies at the 90% confidence level based on the Student s t-test. the slight deviation of the GH positive center in the high latitude regions. The GH negative values in the eastern part of Eurasia are also most remarkable in February. At 500 hpa in the troposphere, the magnitude of GH differences is reduced, but the distribution pattern is similar to that in the upper atmosphere. The GH negative center, which has direct impacts on the temperature distribution in eastern China, becomes strongest in February and moves eastward slightly, with the largest center located in the eastern part of the Eurasian continent. Based on the above analysis of decadal mean GH differences, an apparent decadal variation is found in both the stratosphere and troposphere around the year 2000, and these differences are vertically continuous. In the 2000s, the polar vortex shows more disturbances and a frequent cold vortex center in eastern Eurasia, which has a considerable influence on cold air outbreaks in eastern and northern China. Figure 4a shows the differences in the winter mean

590 JOURNAL OF METEOROLOGICAL RESEARCH VOL.30 circulation and GH at 500 hpa between the 2000s and 1990s. At 500 hpa, a strong positive center of GH differences is located in the Ural Mountains area, and this anomalous anticyclone center covers almost the entire northern Eurasian continent (north of 70 N), extending southward to 30 N, 40 60 E. In the midlatitudes the lower reaches of this positive center there is a negative center corresponding to the location of the East Asian trough, which has considerable influences on the temperature distribution in China. In the distribution of wind fields, anticyclonic winds are observed around the positive center in the Ural Mountains area, and the southerly wind on the western side of this positive center can transport warm air to the high latitudes from the mid and low latitudes. The midlatitude regions, in the lower reaches of the blocking high, are influenced by the northerly wind on the eastern side of the blocking high, leading to a cyclonic pattern in the flow field of these regions. These distribution characteristics indicate that since 2000, the Ural blocking high and East Asian trough both become more active in winter in the Northern Hemisphere. The strong northerly wind on the eastern side of the blocking high can transport cold air southward to the midlatitudes where a cut-off low system may be generated, leading to the gradual accumulation of cold air. When the Ural blocking high collapses, the cold air will break out rapidly eastward and southward with the reinforcement and movement of the East Asian trough, resulting in major cold air outbreaks in eastern and northern China. From the distribution of near-surface temperature differences between the 2000s and 1990s (Fig. 4b), it is clear that strong positive values are situated in the polar areas with intensities mostly exceeding 2 K, while in the midlatitudes of the eastern Eurasian continent, apparent negative anomalies of temperature are observed (with large intensity and wide coverage). Negative anomalies dominate all the regions to the north of 30 N in eastern China. All of the above distribution features indicate that, compared with the 1990s, the winter mean temperature decreases substantially and there are more cold air activities in eastern and northern China after 2000. This is consistent with the conclusions from the EOF analysis, which further proves the presence of decadal variation in the winter circulation of the stratosphere troposphere coupled system. In addition, it is proposed that this variation bears a close relationship with atmospheric cold air activities in China. 4. Relationship between the NAM index and East Asian trough The above analyses show that after 2000, there are more disturbances in the stratospheric polar vortex in boreal winter, and the Ural blocking high and East Asian trough also become more active, leading to enhanced cold air processes in eastern and northern China. Therefore, it is important to explore more specific connections between the stratospheric anomalies and cold air activity in China. Previous studies Fig. 4. Differences in (a) winter mean GH (contours; gpm) and wind fields (arrows; m s 1 ) at 500 hpa and (b) temperature (color-shaded; K) between the 2000s and 1990s at 1000 hpa.

NO.4 LU Chunhui, ZHOU Botao and DING Yihui 591 have shown that variations in the NAM index can reflect development of the stratospheric polar vortex, and it is the key indicator in describing the stratospheric influences on the tropospheric circulation (Kong and Hu, 2014). Therefore, in this study, we select several winters in the 1990s and 2000s as typical cases (displayed in Table 1) to represent the disturbed stratosphere troposphere system. Based on the evolution of the first EOF time series (red solid line in Fig. 2a), six inflection points with minimum values in the 1990s are selected as the typical winters in NAM positive phase, while five inflection points with maximum values in the 2000s are selected as the typical winters in NAM negative phase. In view of the East Asian trough, which is a significant synoptic system and has direct impacts on cold air activities in China, we next discuss the relationship between the NAM index and East Asian trough against different backgrounds of decadal characteristics. In our identified typical years of the 1990s and Table 1. Critical dates of the NAM index at 200 hpa and subsequent intensity variations of the East Asian trough in typical winters in the 1990s and 2000s 1990s Typical First date of The East Typical First date of Date of the Column 6 minus Location of winter NAM > 1 Asian trough winter NAM < 1 strongest trough column 5 (day) the strongest trough 1989 1990 1990.2.1 Weakened 2000 2001 2000.12.5 2000.12.11 6 147.0 E 1992 1993 1992.12.27 Fluctuated 2000 2001 2001.2.13 2001.2.20 7 161.5 E 1994 1995 1995.2.15 Fluctuated 2003 2004 2003.12.26 2003.12.30 4 153.5 E 1995 1996 1996.2.5 Reinforced 2005 2006 2006.3.8 2006.3.13 5 143.0 E 1996 1997 1997.1.1 Weakened 2008 2009 2009.1.16 2009.1.24 8 130.5 E 1999 2000 2000.1.8 Weakened 2009 2010 2010.1.16 2010.1.22 6 152.5 E 2000s 2000s, we define the central date of each case, LagT(0) in the composite, as the date when the absolute value of NAM index at 10 hpa becomes larger than 1. Figure 5 shows the composite distribution of NAM index in boreal winter in the 1990s and 2000s and it can be seen that positive (negative) NAM anomalies, which represent the strong (weak) polar vortex, propagate from the upper stratosphere down to the mid and lower troposphere and remain in place for a long time. Further analyses of the relationship between the NAM index and East Asian trough illustrate that the NAM index at 200 hpa can provide some indication of the intensity variation of the trough in the period of the weak polar vortex (the 2000s). Conversely, for the 1990s, characterized by the strong vortex, it is difficult to find a reliable relationship between the NAM index and East Asian trough. The evolutions of the trough intensity and the NAM index in the five typical winters of the 2000s are shown in Fig. 6. Both time series are standardized to ensure their comparability in different years. If we only analyze the linear relationship between these two time series, the correlation coefficients are not large, indicating that there is no apparent connection between these two factors simply from the statistical perspective. This is because the East Asian trough is a tropospheric synoptic system and has diverse variation features on different timescales. Its development is mainly affected by the tropospheric circulation, and anomalous signals from the stratosphere are unlikely to play a dominant role in the variability of the trough. However, when strong disturbances appear in the stratospheric circulation and propagate down to the troposphere, apparent responses will be observed in the intensity variation of the East Asian trough. As shown in Fig. 6, in all five winters when the NAM index decreases sharply, a corresponding significant decline is also observed in the intensity curves of the trough, indicating the reinforcement of the East Asian trough. Table 1 shows the dates on which the NAM index at 200 hpa reduces below 1, the dates on which the trough intensity reaches a maximum, and the corresponding location of the strongest trough, in typical winters in the 2000s. When negative NAM anomalies at 200 hpa reach a certain intensity, influences from the stratosphere are able to induce the deepening of the East Asian trough, which reaches maximum intensity after about 1 week (Table 1). In contrast, Table

592 JOURNAL OF METEOROLOGICAL RESEARCH VOL.30 Fig. 5. Composite distributions of NAM index in boreal winter in typical years of the (a) 1990s and (b) 2000s. Fig. 6. Evolutions of the intensity of the East Asian trough (blue solid line; left axis) and the NAM index at 200 hpa (red dashed line; right axis) in the winters of (a) 2000 2001, (b) 2003 2004, (c) 2005 2006, (d) 2008 2009, and (e) 2009 2010.

NO.4 LU Chunhui, ZHOU Botao and DING Yihui 593 1 also shows the date when the NAM index at 200 hpa becomes larger than 1 and the subsequent situations of the trough intensity in typical winters in the 1990s. During the NAM positive phase, the polar vortex is strong and stable in the whole atmosphere, representing the features of a cold cyclone center. Downward propagations of the positive NAM index indicate the signal of the strong stratospheric polar vortex, and its major impacts on the lower atmosphere are concentrated in the polar areas, with little influence on the midlatitude system (the East Asian trough). Thus, there is no clear correlation between the positive NAM index and intensity variations of the East Asian trough. Based on the results of Table 1, Fig. 7 shows the composite distributions of GH and its anomalies in the typical winters at 500 hpa, one week after the NAM anomalies propagate to 200 hpa, in the 1990s and 2000s. In the 2000s, when downward propagations of negative NAM anomalies arrive at the upper troposphere and these anomalous signals from the stratosphere reach a certain intensity (standardized NAM index less than 1), the East Asian trough is significantly reinforced. Negative GH anomalies can be clearly observed in the defined regions of the East Asian trough, with the largest GH center reaching about 120 gpm, and the trough location is around 140 E (Figs. 7a and 7b). In the corresponding figure for the near-surface temperature anomalies in the 2000s (Fig. 7d), the northerly air flow is shown to transport cold air southward and eastward from the high latitudes, leading to apparent cold air processes in most parts of eastern and northern China, with the maximum temperature reduction reaching 4 K. The distribution of negative temperature anomalies is shown to expand eastward to the Korean Peninsula. Further analyses of different typical winters indicate that cold air activities begin as soon as the stratospheric NAM anomalies arrive in the troposphere, and the temperature reductions are gradually enhanced with the increase of the trough s intensity. When the trough moves away from the Eurasian continent, the Fig. 7. (a, b) Composite distributions of GH (contours; gpm) and its anomalies (color-shaded; gpm) at 500 hpa, and (c, d) composite anomalies of near-surface temperature (K) in the typical winters of the (a, c) 1990s and (b, d) 2000s. The red boxes in (a, b) indicate the defined range of the East Asian trough.

594 JOURNAL OF METEOROLOGICAL RESEARCH VOL.30 whole cold air process is over. However, in the 1990s (Figs. 7a and 7c), negative GH anomalies dominate the high latitude areas, indicating a strong and stable polar vortex, and positive anomalies are situated in the range of the East Asian trough with a smooth zonal pattern of GH fields in the midlatitudes, representing a weak trough and warming anomalies in most of China. The above composite results show that the downward propagation of negative NAM index can promote the development of the East Asian trough, while influences of the deepening trough on near-surface temperature are also related to the trough location. Table 1 shows that the locations of the trough are different in the 6 processes in the 2000s. The case in January 2009, characterized by the trough being on the westernmost side, and the case in February 2001, characterized by the trough being on the easternmost side, are therefore selected to conduct further comparisons. In February 2001, the East Asian trough is reinforced and negative GH anomalies are mainly located around 160 E, because of the downward propagation of negative NAM anomalies. The trough shifts eastward and the northerly air flow behind the trough transports cold air mostly to the northeast of Japan and the western Pacific basin. Most areas in China are covered by positive temperature anomalies, indicating weak influences of the deepening East Asian trough. However, in January 2009, negative centers of GH anomalies are situated around 130 E, indicating a more western location of the East Asian trough and its increased impacts on China. Cold air outbreaks can be observed in the distributions of the near-surface temperature anomalies in most areas from north to south in eastern China, and stronger temperature drop occurs mainly in eastern and southern coastal areas. The cold air behind the trough has considerable influences on China. Analyzing all six cases in the 2000s (Table 1), we find that the reinforced East Asian trough, caused by the downward propagation of negative NAM index, usually has notable impacts on cold temperature processes in China, except when the trough location shifts too far eastward (only in February 2001). 5. Conclusions and discussion This study analyzes and diagnoses the decadal variation characteristics of the stratosphere troposphere coupled system in recent decades by using NCEP2 reanalysis data. Against the background of this atmospheric circulation, we further explore the effective stratospheric signals during cold air activities related to the weak polar vortex (2000s), and analyze the relationship between the NAM index at different pressure levels and the tropospheric East Asian trough. Both the EOF analysis of GH annual anomalies and the comparison of the decadal mean of the 1990s and 2000s indicate that there are indeed decadal variation features in the entire atmosphere around the year 2000. Meanwhile, the stratospheric negative NAM index is closely related to the intensity variation of the East Asian trough, which has significant effects on cold air outbreaks in boreal winter in China. The main conclusions are as follows. Based on the EOF analysis of the GH anomalies in recent decades, it is found that the first EOF mode mainly reflects the distribution and variation features of the NAM. Positive GH anomalies can be observed in the polar areas from the near surface to the upper stratosphere, corresponding to the NAM negative phase and a weak polar vortex, while in the midlatitudes, negative GH anomalies are distributed around the vortex. At 500 hpa, negative centers of GH anomalies are situated in the northeast of the Eurasian continent and eastern Atlantic, and there is a positive GH center in the Ural Mountains region. On the other hand, the time series of the first EOF mode exhibits an increasing trend during 1989 2011. In the winters of the 1990s, values of the time series are mainly negative, indicating a strong and stable polar vortex, and a weak Ural blocking high and East Asian trough in this period. However, in the winters of the 2000s, the time series values are mostly positive, indicating more disturbances of the polar vortex and a more active blocking high and East Asian trough, which would induce more cold air activities in eastern and northern China. Furthermore, we compare the decadal mean

NO.4 LU Chunhui, ZHOU Botao and DING Yihui 595 between the 1990s and 2000s and find that the major differences in the stratospheric and tropospheric circulations are distributed in the Northern Hemispheric high latitudes. After 2000, SSW events occur more frequently and tropospheric polar circulations are affected by the stratospheric polar vortex directly, while in the midlatitudes the blocking high and East Asian trough are more active, leading to more frequent cold air activities in eastern Asia. These results further support the conclusions of the EOF analysis. Since 2000, with a more disturbed polar vortex, the stratospheric circulations have stronger influences on the lower atmosphere. Our analyses show that impacts of the stratospheric NAM on the East Asian trough are more significant in the negative phase of the NAM. When the negative NAM index propagates from the upper stratosphere to the troposphere (e.g., 200 hpa), and when the anomalies reach a certain intensity (standardized NAM index less than 1), these stratospheric anomalies are able to induce apparent reinforcement of the East Asian trough, which reaches its maximum strength about one week later. The northerly air flow behind the trough can transport cold air southward and eastward from the high latitudes, resulting in enhanced cold air processes in most areas of eastern and northern China. Composite analysis shows that the temperature decrease can reach about 4 K. Further diagnoses of different processes illustrate that the cold air process commences as soon as the downward propagation of NAM anomalies arrives at the troposphere, and the cold event intensity and coverage are closely related to the location of the East Asian trough. In most winters, the deepened trough, caused by the stratospheric NAM anomalies, is able to induce apparent temperature drop in eastern China. However, in February 2001, the trough location shifts too far eastward and the major impact areas are transferred to northeastern Japan and western Pacific basins. Therefore, we conclude that the NAM index (especially at the 200-hPa level) can be used as a stratospheric anomalous signal to provide some indication of the cold air activity in boreal winter in China. Compared with previous research (Thompson and Wallace, 2000; Baldwin and Dunkerton, 2001), the present study uncovers a number of differences between our first EOF mode and the traditional NAM pattern. Further test of the EOF analysis finds that the different time ranges of the data used are the primary reason. NCEP2 reanalysis data from January 1979 to December 2012 are used in this study. As a new version of NCAR reanalysis data, NCEP2 offers a better description of stratospheric circulation. On the other hand, the data used in previous studies are mainly before the year 2000. As we know, after 2000, there have been changes in anthropogenic activities that may have had noticeable influences on the climate system. In particular, emissions of fluoride and chloride have been restricted, so as to protect the stratospheric ozone layer, which has had direct impacts on the distribution and variation of the stratosphere. There have also been some internal variabilities in the atmospheric circulation after the year 2000. Furthermore, our analyses, and some other investigations, such as Wei et al. (2011), all find the occurrence of decadal variation around the year 2000. Therefore, the data after the year 2000 are of great importance in current research and analysis. The NAM pattern may have some different features in the 21st century. The definitions of the intensity and location of the East Asian trough used in this paper are derived from the NCC and have been widely applied in climate monitoring and prediction. However, other definitions could instead be used, which may lead to some differences in the conclusions given above. However, it is clear that downward propagations of negative NAM index are able to enhance the intensity of the East Asian trough. The most important synoptic systems in East Asia are the Ural blocking high and the East Asian trough. A previous study of Kong and Hu (2014) and this study indicate that the stratospheric NAM index has close connections with these two systems, and thus we can combine these two studies to scientifically improve stratospheric predictors of cold air activity in China. In addition, the planetary wave is also a useful dynamic diagnostic tool in investigating the stratospheric influence on the troposphere

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